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SMS PDU mode

The PDU mode offers to send binary information in 7 bit or 8 bit format. That is helpful if you have to send compressed data, binary data or you you like to build your own encoding of the characters in the binary bit stream. If you go back on the old encoding of a Fernschreiber, then there are only 5 bit needed to send an alphanumeric text. By 5 bit coding you can contain 224 characters instatt of 160 characters in 7 bit Text mode. An others reason could be the sending of integer data.

If you would like to have the full control of your transmited data in Text mode you have to understand the PDU mode, because there are a few commands where you can set numeric parameters that change the kind od send and receive of a SMS in text mode also.

Please note that there are a few differences of in the kind of implemetation of the PDU mode and by the other AT commands.

SMS and the PDU format

The SMS message, as specified by the Etsi organization (documents GSM 03.40 and GSM 03.38), can be up to 160 characters long, where each character is 7 bits according to the 7-bit default  alphabet. Eight-bit messages (max 140 characters) are usually not viewable by the phones as text messages; instead they are used for data in e.g.  smart messaging (images and  ringing tones) and OTA provisioning of  WAP settings. 16-bit messages (max 70 characters) are used for Unicode (UCS2) text messages, viewable by most phones. A 16-bit text message of class 0  will on some phones appear as a Flash SMS (aka blinking SMS or alert SMS).

The PDU format

There are two ways of sending and receiving SMS messages:  by text mode and by PDU (protocol description unit) mode. The text mode  (unavailable on some phones) is just an encoding of the bit stream represented by the PDU mode. Alphabets may differ and there are several encoding alternatives when displaying an SMS message. The most common options are  "PCCP437", "PCDN", "8859-1", "IRA" and "GSM". These are all set by the at-command AT+CSCS, when you read the message in a computer application. If you read the message on your  phone, the phone will choose a proper encoding. An application capable of reading incoming SMS messages, can thus use text mode or PDU mode. If text mode is used, the application is bound to (or limited by) the set of preset encoding  options. In some cases, that's just not good enough. If PDU mode is used, any  encoding can be implemented.
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